Marcum, 31, signed a one-year, $4 million deal with the Mets in the offseason. He went 1-10 with a 5.29 ERA in 78 1/3 innings.
He has battled injuries throughout his career, managing more than 30 starts just twice and missing all of the 2009 season after having Tommy John surgery.
As for the injury itself, let us turn to the Mayo Clinic:
Thoracic outlet syndrome is a group of disorders that occur when the blood vessels or nerves in the thoracic outlet -- the space between your collarbone and your first rib -- become compressed. This can cause pain in your shoulders and neck and numbness in your fingers.
It doesn't take a surgeon to figure out why pain in shoulders and neck that is accompanied by numbness in the fingers would severely affect a pitcher's ability to do his job. Chris Carpenter and Mike Adams are two players who have been recently afflicted with this and neither has been able to stay completely healthy for long stretches since.